A fluorescent substance is used in a visual fluorescence display tube (VFD), a field emission display (FED), a plasma display panel (PDP), a cathode ray tube (CRT), a white light emitting diode (LED) and the like. In any of these uses, in order to make a fluorescent substance emit light, it is necessary to provide the fluorescent substance with energy for exciting the fluorescent substance, and a fluorescent substance emits a visible light by being excited by an excitation source having a high energy of a vacuum ultraviolet light, an ultraviolet light, an electron beam, a blue light and the like. However, as a result that a fluorescent substance is exposed to such an energy source as described above, the fluorescent substance is liable to be reduced in luminance and deteriorated in quality, and thus a fluorescent substance being less in reduction of luminance is demanded. Therefore, as a fluorescent substance less in reduction of luminance, a sialon fluorescent substance has been proposed instead of a conventional fluorescent substance such as a silicate fluorescent substance, a phosphate fluorescent substance, an aluminate fluorescent substance, a sulfide fluorescent substance or the like.
An example of such a sialon fluorescent substance is manufactured by a manufacturing process as roughly described below. It is manufactured by first mixing silicon nitride (Si3Ni4), aluminum nitride (AlN) and europium oxide (Eu2O3) together in a specified mole ratio, keeping the mixture at a temperature of 1700° C. in a nitrogen atmosphere of 1 atm. (0.1 MPa) for 1 hour, and burning it by means of a hot press method (see patent literature 1 for example). It has been reported that an α sialon having Eu ions activated which has been obtained by this process becomes a fluorescent substance emitting a yellow light of 550 to 600 nm by being excited by a blue light of 450 to 500 nm.
Further, there are known a blue light fluorescent substance having a JEM phase (LaAl(Si6-zAlz)N10-zOz) as a parent crystal and having Ce activated (see patent literature 2), a blue light fluorescent substance having La3Si8N11O4 as a parent crystal and having Ce activated (see patent literature 3), and a red light fluorescent substance having CaAlSiN3 as a parent crystal and having Eu activated (see patent literature 4).
However, a fluorescent substance emitting a green light in addition to fluorescent substances emitting blue and yellow lights is also demanded for a white LED having an ultraviolet LED as an excitation source, a plasma display and the like.
Reference Literatures
Patent literature 1; Japanese Patent Laid-Open Publication No. 2002-363,554
Patent literature 2; Japanese Patent Application No. 2003-208,409
Patent literature 3; Japanese Patent Application No. 2003-346,013
Patent literature 4; Japanese Patent Application No. 2003-394,855
As another sialon fluorescent substance, a fluorescent substance obtained by adding a rare earth element to a β-type sialon (see patent literature 5) is known, and it has been shown that such a fluorescent substance having Tb, Yb or Ag activated becomes a fluorescent substance emitting a green light of 525 nm to 545 nm. However, since an activated element remains in a grain boundary without being sufficiently solid-dissolved into a crystal due to its synthesizing temperature being as low as 1500° C., a high-luminance fluorescent substance has not been obtained.
Reference Literature
Patent literature 5; Japanese Patent Laid-Open Publication No. Sho 60-206,889